Photolithography is a fundamental integrated circuit fabrication technique that transfers a pattern from a photomask (often referred to as a reticle) to a layer of photoresist. The pattern is transferred by exposing the photoresist layer to light through the photomask, and is then developed to form a resist mask.
As integrated circuits reach ever-higher levels of integration, their shrinking internal dimensions, including both the sizes of circuit elements and the spacing between the circuit elements, pose problems for photolithography. One problem is the optical proximity effect (OPE), a general term for various phenomena that distort the mask pattern when it is transferred to the photoresist. For example, right-angled corners may become rounded, lines may become shortened at their ends, and line widths may be altered.
These optical proximity effects can seriously alter the electrical characteristics of integrated circuits. They also affect fabrication yields, by causing unintended metal lines to overlap, for example, leading to short circuits and device failure.
Known methods of compensating for optical proximity effects include the optical proximity corrections disclosed in U.S. Pat. No. 5,723,233 to Garza et al., and U.S. Pat. No. 5,879,844 to Yamamoto et al. These known methods include increasing the contrast of the exposure apparatus, expanding the process window, adding auxiliary patterns or serifs to the main mask pattern, and increasing or reducing the line widths in the main mask pattern.
The known corrective methods, however, fail to provide a simple method adequate to the needs of current high levels of integration. It is the object of the present invention to produce a square contact/via or line-end pattern at the end of the line as it appears on the mask.